For example, an uninterruptable power supply device includes a storage battery or a capacitor as a power storage unit. A rated voltage per cell of the storage battery or capacitor is, for example, about 2V to 3V. A large number of cells are connected in series, so that a charging and discharging voltage of the power storage unit is adjusted. The plurality of cells connected in series are also referred to as a power storage module. A plurality of power storage modules are connected in parallel to secure a required capacity of the power storage unit.
Occurrence of an abnormality is concerned in the power storage unit. For example, a resistance value of a cell increases due to aged deterioration. Or, ground fault may occur in which a cell short-circuits to a ground potential. Therefore, there has been conventionally proposed an abnormality detection circuit for detecting such an abnormality, and a protection circuit for preventing extension of such an abnormality.
For example, a storage battery ground fault detection circuit disclosed in Japanese Patent Laying-Open No. 7-146321 (PTD 1) is used for an uninterruptable power supply device. This uninterruptable power supply device includes an alternating current power supply, a rectifier, an inverter, and a storage battery. The alternating current power supply is grounded at its one end. The rectifier is connected to the alternating current power supply and converts alternating current power into direct current power. The inverter converts direct current power outputted from the rectifier into alternating current power. The storage battery is connected to a direct current intermediate circuit coupling a direct current side of the rectifier and a direct current side of the inverter. In such an uninterruptable power supply device, an alternating current ground current detection means for detecting an alternating current ground current is provided on an alternating current side of the rectifier.
Japanese Patent Laying-Open No. 2002-159135 (PTD 2) discloses a ground fault protection circuit for an electric double layer capacitor module. The electric double layer capacitor module is constituted of a plurality of cells connected in series. This ground fault protection circuit includes current detection means and a breaker. The current detection means is provided on a ground terminal side of the electric double layer capacitor module. The breaker is provided in series on the ground terminal side of the electric double layer capacitor module and is opened based on a current value detected by the current detection means.